An optical waveguide mixes and directs light emitted by one or more light sources, such as one or more light emitting diodes (LEDs). A typical optical waveguide includes three main components: one or more coupling surfaces or elements, one or more distribution elements, and one or more extraction elements. The coupling component(s) direct light into the distribution element(s), and condition the light to interact with the subsequent components. The one or more distribution elements control how light flows through the waveguide and such control is dependent on the waveguide geometry and material. The extraction element(s) determine how light is removed by controlling where and in what direction the light exits the waveguide.
When designing a coupling element, the primary considerations are: maximizing the efficiency of light transfer from the source into the waveguide; controlling the location of light injected into the waveguide; and controlling the angular distribution of the light in the waveguide. The coupling element of a waveguide may be comprised of one or more of a number of optical elements, including a primary source optic (such as the lens on an LED component package), one or more intermediate optical elements (such as a lens or array of lenses) interposed between the source(s) and the waveguide coupling surface or surfaces, one or more reflective or scattering surfaces surrounding the sources, and specific optical geometries formed in the waveguide coupling surfaces themselves. Proper design of the elements that comprise the coupling element can provide control over the spatial and angular spread of light within the waveguide (and thus how the light interacts with the extraction elements), maximize the coupling efficiency of light into the waveguide, and improve the mixing of light from various sources within the waveguide (which is particularly important when the color from the sources varies—either by design or due to normal bin-to-bin variation in lighting components). The elements of the waveguide coupling system can use refraction, reflection, total internal reflection, and surface or volume scattering to control the distribution of light injected into the waveguide.
It is desirable to maximize the number of light rays emitted by the source(s) that impinge directly upon the coupling surface in order to increase the coupling of light from a light source into a waveguide. Light rays that are not directly incident on the waveguide from the source must undergo one or more reflections or scattering events prior to reaching the waveguide coupling surface. Each such ray is subject to absorption at each reflection or scattering event, leading to light loss and inefficiencies. Further, each ray that is incident on the coupling surface has a portion that is reflected (Fresnel reflection) and a portion that is transmitted into the waveguide. The percentage that is reflected is smallest when the ray strikes the coupling surface at an angle of incidence relative to the surface normal close to zero (i.e., approximately normal to the surface). The percentage that is reflected is largest when the ray is incident at a large angle relative to the surface normal of the coupling surface (i.e., approximately parallel to the surface).
In one type of coupling, a light source that emits a Lambertian distribution of light is positioned adjacent to the edge of a planar waveguide element. The amount of light that directly strikes the coupling surface of the waveguide in this case is limited due to the wide angular distribution of the source and the relatively small solid angle represented by the adjacent planar surface. To increase the amount of light that directly strikes the coupling surface, a flat package component such as the Cree ML-series or MK-series (manufactured and sold by Cree, Inc. of Durham, N.C., the assignee of the present application) may be used. A flat package component does not include a primary optic or lens formed about an LED chip. A flat emitting surface of the flat package component may be placed in close proximity to the coupling surface of the waveguide. This arrangement helps ensure a large portion of the emitted light is directly incident on the waveguide.
After light has been coupled into the waveguide, it must be guided and conditioned to the locations of extraction. In accordance with well-known principles of total internal reflection light traveling through a waveguide is reflected back into the waveguide from an outer surface thereof, provided that the incident light does not strike the outer surface at an angle less than a critical angle with respect to the surface. Specifically, the light rays continue to travel through the waveguide until such rays strike an index interface surface at a particular angle less than an angle measured with respect to a line normal to the surface point at which the light rays are incident (or, equivalently, until the light rays exceed an angle measured with respect to a line tangent to the surface point at which the light rays are incident) and the light rays escape.
In order for an extraction element to remove light from the waveguide, the light must first contact the feature comprising the element. By appropriately shaping the waveguide surfaces, one can control the flow of light across the extraction feature(s) and thus influence both the position from which light is emitted and the angular distribution of the emitted light. Specifically, the design of the coupling and distribution surfaces, in combination with the spacing (distribution), shape, and other characteristic(s) of the extraction features provide control over the appearance of the waveguide (luminance), its resulting light distribution (illuminance), and system optical efficiency.
Light extracting elements have been designed that can be applied to a waveguide element to obtain a desired illuminance distribution. Such elements are disclosed in U.S. patent application Ser. Nos. 14/472,078 and 14/472,064, owned by the assignee of the present application and the disclosures of which are hereby incorporated by reference herein. Such light extracting elements are disposed on one or more sheets of transparent material that are, in turn, secured by a transparent adhesive to a waveguide element. While a waveguide manufactured using such a process is effective to produce a desired illumination distribution, use of an adhesive reduces efficiency and imposes an extra step and expense into the production resulting in decreased throughput and increased cost.